1. Field of the Invention
Example embodiments of the present invention relate to methods for forming a multi-shell nanocrystal structure, multi-shell nanocrystal structures and a method for fabricating a device including the same. Other example embodiments of the present invention relate to methods for forming a multi-shell nanocrystal structure by preparing a core nanocrystal and simultaneously using at least two precursors having different reaction rates to sequentially form at least two shell layers of nanocrystals having different compositions on a surface of the core nanocrystal, multi-shell nanocrystal structures and fabricated device including the same.
2. Description of the Related Art
A nanocrystal designates a material of a nanometer-sized crystal composed of several hundreds to several thousand atoms. A nanocrystal exhibit electrical, magnetic, optical, chemical and mechanical properties different from those inherent properties of the materials itself. The properties of nanocrystals could be variously adjusted by controlling the physical size of nanocrystals.
Vapor deposition processes, (e.g., metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE)) has been generally used to prepare nanocrystals. A wet chemistry technique, wherein a precursor material may be added to an organic solvent to grow a nanocrystal, has been rapidly developed recently. In the wet chemistry technique, a surfactant is coordinated on the surface of the nanocrystal to control the crystal growth during reaction. Therefore, the wet chemistry technique could provide nanocrystals of more uniform size and shape with a lower cost, compared to other conventional vapor deposition processes (e.g., MOCVD, MBE, etc.).
Conventional processes (U.S. Pat. Nos. 6,225,198 and 6,306,736) provide a technique for forming nanocrystals by adding a Group II-VI or Group III-V precursor in a surfactant and a solvent at crystal growth temperature.
The conventional art (U.S. Pat. Nos. 6,322,901 and 6,207,229) discloses a core-shell nanocrystal of a type I structure and a method for preparing the nanocrystal, exhibiting enhanced quantum efficiency about 30-50% with maintaining the emission wavelength of a core nanocrystal.
The core-shell nanocrystal structure is generally prepared by preparing and separating a core and introducing the separated core into a reaction mixture to grow shell on surface of core.
In the above-mentioned conventional techniques, the optical property of nanocrystal emitting pure-wavelength light with high efficiency, which induces by the electron-hole recombination only at an edge of an energy band gap, may be applied to displays or to bio image sensors.
In addition, the conventional art (U.S. Patent Publication No. 2004/011002) discloses a method for preparing core-shell nanocrystal structure having a type II structure. In the conventional nanocrystal structure of type II, which has higher conduction and valence bands of a core than a shell layer or lower conduction and valence bands of a core than a shell layer, excited electrons and holes are dispersed and confined in the core and the shell layer, respectively. Therefore, the emission wavelength of the type II nanocrystal structure may be shifted to a longer wavelength with decrease of the quantum efficiency.